Stirling-cycle engines, sometimes called hot-air engines or hot-gas engines, are well known, as evidenced by more than 600 original and related patents, and are a genre of machines chiefly characterized by an operating process in which an internally contained quantity of gas is alternately and periodically heated and cooled, via conduction, through the walls of parts of the machine, by an external heat source and an external heat sink, in order to perform work.
These engines are attractive because of their high theoretical efficiency, their ability to utilize a large variety of fuels or energy sources, and their potential for reducing hydrocarbon emissions.
The chief difficulties inhibiting the commercial exploitation of these engines presently are that they must have high internal pressures in order to have a high specific output or efficiency or power-to-weight ratio; they must have high temperature, high pressure, dynamic seals to prevent escape of the working gas, and to separate machine lubricants from the working gas, because such mixing or contamination reduces the efficiency of the engines, and these requirements have heretofore resulted in unacceptable expense in the manufacture of these engines. Higher pressures usually result in heavier machines because the machine sections must be stronger in order to contain the internal forces, and as the engines increase in size in order to achieve higher horsepower, the weight and cost of these sections becomes impractical.
In addition to being a pressure vessel, the Stirling-cycle engine must maintain the greatest possible temperature differential between the hot side and the cold side of the engine. Therefore, conduction through the engine body and working parts must be minimized through selection of materials and through designs which reduce conductive pathways.
Lastly, the Stirling, in order to compete successfully with existing engines, must be cheap to produce, with no major retooling or special production equipment, and it must be reasonably easy to maintain, and have a service life comparable to existing internal-combustion engines of equivalent performance.
These and other considerations, as well as a history of the technical development of Stirling engines are set forth in a book by G. Walker entitled, "Stirling Cycle Machines", and by the Philips Technical Review, vol. 31, "Prospects of the Stirling engine for Vehicular Propulsion".